This invention relates to color cathode ray tubes having aperture masks, and more particularly relates to improved internal magnetic shields for such tubes.
Color cathode ray tubes for color television (color picture tubes) employing aperture masks are known to be sensitive to external magnetic fields, especially the earth's magnetic field, which can undesirably influence the trajectories of the electron beams, causing shifting and distortion of the screen raster, as well as degradation of color purity and convergence. For example, shifting of a beam trajectory to the right or left will result in a beam landing error in a tube with vertically oriented phosphor stripes. That is, the beam will land to the right or left of the intended landing area on the stripe.
For acceptable performance, therefore, such tubes must have effective shielding from these magnetic fields, and it has become conventional practice to incorporate internal magnetic shields into color picture tubes for this purpose.
Because the effect of the earth's magnetic field depends upon the location and orientation of the tube, optimum shielding requires the ability to remagnetize the shield to realign the magnetic domains after the tube has been moved. Accordingly, these shields are customarily fabricated from a soft magnetic material, such as low carbon steel, enabling ready remagnetization each time the television set is turned on.
The shape of the shield is in general dictated by the desire to have as much of the tube volume shielded as possible, without having the shield interfere with the tube's operation. For example, extending the shield too far back into the funnel risks interference with the magnetic deflection field for the electron beams, as well as physical interception or "clipping" of the deflected beams. In addition, cost considerations dictate as simple a shape as possible.
In Japanese patent No. 52-18165, the shield is composed of two curved sheets of magnetic material, said to make processing, storing, transportation and assembly convenient and easy. In U.S. Pat. No. 3,867,668, a similar internal shield is shown, but in addition an external shield is employed to cover the gaps created by the opening between the two curved sheets "to provide complete shielding".
Recently, despite their complex shapes, bowl-shaped internal magnetic shields have been readily formed by a process in which a foil sheet of shielding material is forced over an inverted bowl-shaped mold, resulting in the material being stretched and crimped into the desired shape.
It is customary to describe the effect of the earth's magnetic field on beam landing in terms of two components; the east-west effect, determined by a transverse horizontal field, and the north-south effect, determined by an axial horizontal field. The magnitude of these effects is the difference between the beam landing errors in the east and west-facing directions, and the north and south-facing directions, respectively. It is also customary to describe the tube's surface in terms of the face of an analog clock. Thus, the top is 12 o'clock, the bottom is 6 o'clock, the right side (as seen by the viewer) is 3 o'clock and the left side is 9 o'clock.
In one recent design, shielding of the east-west effect is reported to be improved by introducing triangular-shaped openings, so-called "balancing windows", into the 6 and 12 o'clock sides of the internal shield. (Toshiba Product Information sheet). The shield is fabricated from two flat sheets of steel, which are cut to form the desired outlines and openings, subjected to a series of straight bends to form the desired shapes, and assembled. While such improved shielding is desirable, the added costs associated with the forming and assembling of these shields is a serious concern in the present cost-competitive environment.
Accordingly, it is an object of the invention to provide improved internal magnetic shields for color tubes which exhibit improved shielding and which can be readily fabricated using present manufacturing techniques.